linux-kernel - Reversing a memory mapping?

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Message-ID: <87k4hgn8qa.fsf@inspiron.ap.columbia.edu>
Date:	Thu, 03 Feb 2011 15:50:21 -0500
From:	Nikolaus Rath <Nikolaus@...h.org>
To:	linux-kernel@...r.kernel.org
Subject: Reversing a memory mapping?

Hello,

I am trying to outsmart the proprietary nvidia driver to get my own
driver to access to a memory region set up by the nvidia driver.

Strace tells me that the nividia userspace library is calling

mmap2(NULL, 1048576, PROT_READ|PROT_WRITE, MAP_SHARED, 5, 0x35375)

to get a userspace pointer to the kernel memory that I'm interested in.

Is there a way that my own kernel module can "reverse" this mmap call?
I.e. given the userspace pointer, how do I get access to the physical
location of the memory that has been mapped?

I know that the region is pinned and used as a DMA buffer for
communication with the GPU. I want my own device to do DMA transfers to
and from the same location.

Thanks,

   -Nikolaus

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